The invention relates to a flat gasket for sealing a sealing gap between sealing surfaces facing one another on machine components containing several passages for hot fluids (such as exhaust gases of internal combustion engines), which open into fluid ports formed at locations opposite one another in the machine component sealing surfaces, a gasket plate of the flat gasket which is to be clamped between the machine component sealing surfaces and comprises at least one sheet metal layer having for each pair of fluid ports located opposite one another on the two machine components a fluid passage opening, associated with which is at least one sealing element of the gasket plate, which encloses the fluid passage opening, and the gasket plate having adjacent to the fluid passage openings several screw holes for the passage of screws serving to clamp the flat gasket between the machine component sealing surfaces.
The invention further relates to an assembly with a cylinder head and an exhaust manifold having a flange which is joined to the cylinder head by means of screws which each have a threaded shaft, and with a flat gasket clamped between the cylinder head and the exhaust manifold flange.
In the following the problem underlying the invention will be explained with reference to an exhaust manifold gasket for a modern internal combustion engine, although the invention is of advantage to all systems in which a flat gasket is clamped between two machine components which have substantially different temperatures while the machine is in operation or during an operational phase.
When a cold internal combustion engine is put into operation, the flange of an exhaust manifold which is to be connected to a cylinder head by screws with the interposition of an exhaust manifold gasket exhibits considerable displacements in the plane of the so-called sealing gap defined by the sealing surfaces of the cylinder head and the exhaust manifold flange accommodating the exhaust manifold gasket between them. The causes of the movements of the sealing surfaces of cylinder head and exhaust manifold flange relative to one another are to be seen in the following: The high exhaust gas temperatures (up to approximately 900° C.) heat up the exhaust manifold quickly, whereas the cylinder head remains relatively cool on account of it being cooled by the coolant of the internal combustion engine, and the exhaust manifold gasket impedes heat exchange between cylinder head and exhaust manifold, which results in a high temperature gradient between these two machine components. Furthermore, in these modern engines the materials of cylinder head and exhaust manifold have quite different coefficients of thermal expansion, as the cylinder head consists of a light metal alloy, whereas the exhaust manifold is a welded construction of steel parts or a gray cast iron part. It must, however, be ensured that in each operational phase of the internal combustion engine the sealing gap between cylinder head and exhaust manifold flange is reliably sealed by the exhaust manifold gasket.
In this connection, particular heed must be paid to the connection between cylinder head and exhaust manifold, which is made by means of screws with a screw head screwed into threaded bores of the cylinder head or by threaded bolts with nuts screwed onto these. In this respect, two extreme constructions will first be considered:
(1) rigid connection of the exhaust manifold with the cylinder head, in particular, by the above-mentioned screws or nuts being tightened with a very high tightening torque, and
(2) connection of the exhaust manifold with the cylinder head such that exhaust manifold flange and cylinder head are displaceable relative to one another.
The rigid connection (1) requires a firm exhaust manifold flange which is followed by a relatively flexible area of the exhaust manifold which can elastically absorb the thermal expansions which occur. Such partially flexible exhaust manifolds are often found in racing car engines with a so-called high-performance header in the form of a welded construction in which several tubes lead from an exhaust manifold flange to a common exhaust pipe. However, such a construction occupies a relatively large constructional space and is also complicated to manufacture. As the different thermal expansions of cylinder head and exhaust manifold lead to the aforementioned screws or threaded bolts being subjected to shearing and tensile stresses, the rigid connection (1) in high-performance headers results in the exhaust manifold flange being subjected to high bending stresses and often also undergoing plastic deformation while the engine is running, which results in an uneven sealing surface of the exhaust manifold flange.
The connection (2) permitting displacements is often found in exhaust manifolds in the form of castings since with a rigid connection (1) the relatively thick-walled castings lead to enormous thrust forces while the engine is running, which can shear off screw or threaded shafts or destroy the threads of the threaded bores provided in the cylinder head. Representative of such a type of connection is the solution already put into practice by ElringKlinger AG, which will be explained in greater detail hereinbelow with reference to FIG. 1. In this construction, the connection between exhaust manifold and cylinder head is made with long screws whose heads are not supported directly on the exhaust manifold flange, but on sleeves which are penetrated by the screw shafts and which themselves, in turn, are supported on the exhaust manifold flange. The screw shafts penetrate the sleeves and the exhaust manifold flange with radial play with respect to the screw shaft axis so that sliding movements between exhaust manifold flange and cylinder head can be elastically absorbed by the screw shafts—the long screw shafts can bend elastically. However, this solution to the problem requires additional components, namely the sleeves, a higher assembly expenditure and a larger constructional space (owing to the screws and sleeves projecting significantly beyond the exhaust manifold flange).
The object underlying the invention was to create a flat gasket for the aforementioned purposes, which, on the one hand, allows sliding movements between the machine components (e.g. cylinder head and exhaust manifold), but, on the other hand, is employable more cost-effectively than the above-described known construction with long screws and sleeves.